I got tired of hearing "not fair" last weekend, and I told the kids they could no longer say that (or any version thereof). And if they could manage it for a week, we'd go out for ice cream. This actually worked out in more way than one, since we were likely to get some requests for ice cream more than once this week. Not they are focused on not saying "no fair" and earning that ice cream.
C5 has been quite good, and has said it only once. She is biting her tounge if need be (although, actually, she was never that bad).
Q4 has said "not fair" twice in my hearing, and once out of ear shot (but, luckily C5 told on her). And this afternoon I was telling them no talking or banging in quiet time, only reading, and she started to say the whiney words and BOOM stopped at "NOT --" and laughed at herself. Yippee!
I've moved the ice cream date from Saturday to Monday. So only a few more days and hopefully they'll have forgotten those words.